After unlocking the pose switch button, which happens after mindbreaking the girl 3 times = reaching full lust (the pink circle around the heart) and her eyes turning to hearts.
After mindbreaking once, the player either needs to lower the lust down to 0 (by keeping the girl in choked state) or restart the game, then mindbreaking can be done again.
Note: 1 mindbreak = Shop, 2 = Autoplay, 3 = Pose siwtch, 4 = Aphrodisiac
P.S.: More usefull info can be found in help popup (tap the ? button in top right corner, then tap on > or on the popup itself to scroll thru help pages)
I hope this helps, cheers
~NK